Question: A major customer of ours, for whom we previously only galvanized steel parts, asked whether we could galvanize and blue passivate a new, large series of zinc die-cast parts in the drum. The only system in our company where this seems possible is a barrel system with a weakly acidic zinc electrolyte (pH 4.8-5.2). The main problem is the pre-treatment: it is designed for steel parts and is correspondingly aggressive. It consists of a highly alkaline decoction degreasing, a hydrochloric acid pickling, an electrolytic degreasing and a hydrochloric acid-based decapitation. After pre-treatment, the parts are transported to the zinc line by a cross transfer unit. The cross-transfer unit is a large tank filled with circulating water.
The reason why we are considering this system is the control system. Our other barrel systems are purely cycle-controlled, so we can't skip baths. The system with the weakly acidic zinc electrolyte, on the other hand, has free movement and is therefore very flexible. We would like to sample an article for the customer, but are unsure about the process sequence. What procedure can you recommend?
Answer: Although the electroplating of zinc die castings has been dealt with intensively in theory, it is still a major issue today. Many workpieces are not coated in contract electroplating stores, but in company electroplating stores, for various reasons. Ideally, casting, pre-treatment and coating are very closely coordinated in order to keep the reject rate as low as possible. Communication between the foundry and the electroplating shop also plays an important role here.
Problem definition
In essence, the coating of zinc die-castings revolves around three problems:
The metal
Zinc die casting is a so-called Zamak alloy and consists of approx. 90-95 % zinc. The potential of zinc is -0.76 V. It is an amorphous metal and is therefore equally attacked by acids and bases. In contrast to chromium, titanium or aluminium, it does not form a very dense oxide layer (protective layer/barrier layer), which facilitates electroplating but makes pre-treatment more difficult. With normal pre-treatment, as used for steel, hardly any of the material remains before electroplating because it is dissolved in the degreasing agents and especially the pickling agent. To put it bluntly, this pre-treatment can also be described as a "zinc die-casting destruction machine".
The alloy
The alloys consist of aluminum, copper and magnesium. They often contain traces of other metals such as lead, cadmium and tin. It is important to ensure that the composition is kept as constant as possible and that the alloy is as homogeneous as possible. Fluctuations in the alloy can have a negative effect on the electroplating result, for example due to uneven surface attack or the formation of poorly soluble salts.
The casting skin
Zinc die casting forms a thin casting skin on which electroplating takes place. Depending on the material (alloy, casting process, part geometry, etc.), the surface may have more or fewer pores, streaks and cold welds, which must be cleaned during pre-treatment to such an extent that an adhesive layer can be deposited. The difficulty lies in finding the right balance. You must clean the surface sufficiently without breaking through the casting skin. If this happens, more pores are exposed. Electrolyte residue often remains in these pores, which leads to the formation of bubbles.
How good the surface condition is depends on the spraying process. We strongly advise you to archive five to ten raw parts per job when coating mass-produced goods. This is the only way you can include the initial condition when investigating the cause in the event of a later complaint or investigation. In the case of coated parts, it is no longer possible to determine whether numerous defects were caused by electroplating or were already present.
Mechanical stress also has an influence on the casting skin. When loading the drums, care must be taken to ensure that the drop height is as low as possible so as not to damage the surface. This aspect should also be pointed out to the customer or foundry.
Pre-treatment
There are specially formulated, mild solutions for the pre-treatment of zinc die castings. As a rule, the barrel material is hot degreased, electrolytically degreased and then decapped. For your process, this means that you would have to swap the two degreasing processes in the system and skip the pickling. If this is not economical or not feasible for other reasons, you can test the following sequence, provided the control system allows this:
The parts can then be coated.
It is advisable to clearly define the delivery condition with the customer. This applies to the containers in general (for optimum handling in the electroplating shop) as well as dirt-free containers in order to obtain parts that are as clean as possible.
Galvanizing
Another critical point is the cross converter. On the one hand, because the water quality can fluctuate and, on the other, because the drums remain in it for several minutes before they are collected, depending on the system. This means that even with the best pre-treatment, we end up with a surface that is in an undefined state.
The drum rotation counteracts this somewhat by mechanically cleaning the surface. Nevertheless, it can happen that residues from the decapping or other impurities penetrate into the pores. To counteract this, no current is applied to the product for the first five minutes in the zinc electrolyte. The surface is cleaned further and the pores are rinsed out with the electrolyte. After five minutes, the current is switched on and electroplating continues until the required layer thickness is reached. If the calculation is based purely on the dwell time, the time without current must be added up.
Further practical information
The pH value of the electrolyte is adjusted or kept constant with hydrochloric acid. This adjustment should be made before coating. When switching from steel to zinc die casting, the system must be emptied in order to change the decapping and, if necessary, restart the rinsing. As soon as the zinc stations are empty, the pH value is set to the lower value (4.8) and should not be corrected during the galvanizing of die-cast zinc. Should this nevertheless be necessary, the dosing of hydrochloric acid must be very slow so that the acid can dilute sufficiently before there is a risk of the material being attacked while the drum is not energized.
Do not measure the pH value with pH paper, but with a calibrated electrode. Some pH papers do not show the pH value correctly in weakly acidic zinc electrolytes. The difference can be up to one pH.
Except in the case of very unfavorable part geometry, it can be assumed that the surface has been completely covered with a zinc layer, which is why no explicit attention needs to be paid to anything during post-treatment. Clean rinsing to achieve the best possible result is nevertheless advisable here. Especially with zinc die-cast and similar materials, it is better to have a spotless surface so that any defects such as bubbles can be easily detected during the final inspection.
